Supernova observed colliding with its companion star

In this still from a simulation, a Type Ia supernova explodes (dark brown colour). The supernova material is ejected outwards at a velocity of about 10,000 kilometres/second. The ejected material then slams into its companion star (light blue colour). The violent collision produces an ultraviolet pulse that is emitted from the conical hole carved out by the companion star. Image credit: Daniel Kasen.
The origin of type Ia supernovae, the standard candles used to reveal the presence of dark energy in the universe, is one of astronomy’s most beguiling mysteries. Astronomers know they occur when a white dwarf explodes in a binary system with another star, but the properties of that second star — and how it triggers the explosion — have remained elusive for decades.

Now, a team of astronomers from the intermediate Palomar Transient Factory (iPTF), including those associated with UC Santa Barbara, have witnessed a supernova smashing into a nearby star, shocking it, and creating an ultraviolet glow that reveals the size of the companion. The discovery involved the rapid response and coordination of iPTF, NASA’s Swift satellite and the new capabilities of the Las Cumbres Observatory Global Telescope Network (LCOGT).

The supernova, named iPTF14atg, is located 300 million light-years away in the galaxy IC 831. The study, appearing in the May 21st issue of Nature, was led by graduate student Yi Cao of Caltech, but included physics postdoctoral fellows Iair Arcavi and Stefano Valenti, and physics faculty member Andrew Howell of UCSB and LCOGT.

In a type Ia supernova, a white dwarf star explodes after it gains matter from a companion star in the same binary star system. One of the leading theories is that the supernova happens when two white dwarf stars merge. But a competing theory says that the companion could be a normal or giant star that survives the explosion, although not without some damage. The supernova is expected to hit the companion star, creating a shock wave that glows in ultraviolet light. This had been theorised in 2010, but such an effect had never been seen. This and other factors led many to conclude that most type Ia supernovae arise from the mergers of two white dwarf stars.

“As you can imagine, I was fired up when I first saw a bright spot at the location of this supernova in the ultraviolet image,” first author Yi Cao said of seeing the ultraviolet flash. “I knew this was likely what we had been hoping for.”

LCOGT, a global network of robotic telescopes, was influential in obtaining early and regular data, allowing the researchers to determine the type and even the strange subclass of the supernova. Initially, the team was puzzled, said Arcavi.

“Hot, blue supernovae are not supposed to happen in old, dead galaxies,” he said. “And yet, as our robotic telescopes gathered the data, we watched in amazement as the blue supernova morphed into a type Ia supernova.”

Upon hearing about the supernova, the LCOGT team immediately triggered their worldwide fleet of robotic telescopes. As the Earth rotated, data was collected at different sites, depending on where it was nighttime and the observing conditions were ideal. Ultimately they combined data from LCOGT telescopes located in Texas, Hawaii and South Africa with data from Palomar and NASA’s Swift satellite to piece together the story of the supernova.

“As the data came in, I started to notice that this supernova was a weird one,” said Valenti. “It was a type Ia, but one with a slow-moving explosion.”

According to the researchers, the supernova belongs to a subclass of SNe Ia sometimes called SN 2002cx-like. These supernovae may even be partially failed or incomplete explosions. In a normal type Ia the entire white dwarf blows up, but this class may leave a piece behind.

There have been conflicting observations about the progenitors of type Ia supernovae. The new study builds on previous work by Howell and some of the study’s coauthors showing that the type Ia SN 2011fe was likely the result of a merger of two white dwarf stars, while the SN Ia PTF11kx seemed to have a red giant companion star.

Said Howell, “No wonder we’ve been so confused for decades. Apparently you can blow up stars in two different ways and still get nearly identical explosions.”

In fact, the study complements work by another postdoc and member of the supernova team at LCOGT and UCSB, Curtis McCully, who was not involved in the present study. He led a team of astronomers who announced in Nature in 2014 that they had found a progenitor on pre-explosion images from the Hubble Space Telescope for a similar SN 2002cx-like supernova, SN 2012Z. In that case, they think what they saw was the companion star, the star that in the case of iPTF14atg shocked the supernova.

“We are finally beginning to see how differences in the progenitor stars relate to differences in the explosion,” McCully said. “This is exciting because the better we understand the origin of type Ia supernovae, the better we can use them as standard candles for cosmology.”

The iPTF project is a scientific collaboration between Caltech; Los Alamos National Laboratory; the University of Wisconsin-Milwaukee; the Oskar Klein Center in Sweden; the Weizmann Institute of Science in Israel; the TANGO Program of the University System of Taiwan; and the Kavli Institute for the Physics and Mathematics of the Universe in Japan. The Caltech team is funded in part by the National Science Foundation.

LCOGT is a global network of 11 one-meter and two-meter telescopes with headquarters in Santa Barbara, California. It has telescopes in Hawaii, Texas, Australia, South Africa and Chile.

NASA’s WISE spacecraft discovers most luminous galaxy in universe

This artist's concept depicts the current record holder for the most luminous galaxy in the universe.

A remote galaxy shining with the light of more than 300 trillion Suns has been discovered using data from NASA’s Wide-field Infrared Survey Explorer (WISE). The galaxy is the most luminous found to date and belongs to a new class of objects recently discovered by WISE — extremely luminous infrared galaxies (ELIRGs).

“We are looking at a very intense phase of galaxy evolution,” said Chao-Wei Tsai of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. “This dazzling light may be from the main growth spurt of the galaxy’s black hole.”

The brilliant galaxy, known as WISE J224607.57-052635.0, may have a behemoth black hole at its belly, gorging itself on gas. Supermassive black holes draw gas and matter into a disk around them, heating the disk to roaring temperatures of millions of degrees and blasting out high-energy, visible, ultraviolet, and X-ray light. The light is blocked by surrounding cocoons of dust. As the dust heats up, it radiates infrared light.

Immense black holes are common at the cores of galaxies, but finding one this big so “far back” in the cosmos is rare. Because light from the galaxy hosting the black hole has traveled 12.5 billion years to reach us, astronomers are seeing the object as it was in the distant past. The black hole was already billions of times the mass of our Sun when our universe was only a tenth of its present age of 13.8 billion years.

The new study outlines three reasons why the black holes in the ELIRGs could have grown so massive. First, they may have been born big. In other words, the “seeds,” or embryonic black holes, might be bigger than thought possible.

“How do you get an elephant?” asked Peter Eisenhardt from JPL. “One way is start with a baby elephant.”

The other two explanations involve either breaking or bending the theoretical limit of black hole feeding called the Eddington limit. When a black hole feeds, gas falls in and heats up, blasting out light. The pressure of the light actually pushes the gas away, creating a limit to how fast the black hole can continuously scarf down matter. If a black hole broke this limit, it could theoretically balloon in size at a breakneck pace. Black holes have previously been observed breaking this limit; however, the black hole in the study would have had to repeatedly break the limit to grow this large.

Alternatively, the black holes might just be bending this limit.

“Another way for a black hole to grow this big is for it to have gone on a sustained binge, consuming food faster than typically thought possible,” said Tsai. “This can happen if the black hole isn’t spinning that fast.”

If a black hole spins slowly enough, it won’t repel its meal as much. In the end, a slow-spinning black hole can gobble up more matter than a fast spinner.

“The massive black holes in ELIRGs could be gorging themselves on more matter for a longer period of time,” said Andrew Blain of the University of Leicester in the United Kingdom. “It’s like winning a hot-dog-eating contest lasting hundreds of millions of years.”

More research is needed to solve this puzzle of these dazzlingly luminous galaxies. The team has plans to better determine the masses of the central black holes. Knowing these objects’ true hefts will help reveal their history, as well as that of other galaxies, in this very crucial and frenzied chapter of our cosmos.

WISE has been finding more of these oddball galaxies in infrared images of the entire sky captured in 2010. By viewing the whole sky with more sensitivity than ever before, WISE has been able to catch rare cosmic specimens that might have been missed otherwise.

The new study reports a total of 20 new ELIRGs, including the most luminous galaxy found to date. These galaxies were not found earlier because of their distance, and because dust converts their powerful visible light into an incredible outpouring of infrared light.

“We found in a related study with WISE that as many as half of the most luminous galaxies only show up well in infrared light,” said Tsai.

Herschel and Planck find missing clue to galaxy cluster formation

The Planck all-sky map at submillimeter wavelengths (545 GHz). The band running through the middle corresponds to dust in our Milky Way Galaxy. The black dots indicate the location of the proto-cluster candidates identified by Planck and subsequently observed by Herschel. The inset images showcase some of the observations made by Herschel’s SPIRE instrument; the contours represent the density of galaxies.

By combining observations of the distant universe made with the European Space Agency’s (ESA) Herschel and Planck space observatories, cosmologists have discovered what could be the precursors of the vast clusters of galaxies that we see today. 

Galaxies like our Milky Way with its 100 billion stars are usually not found in isolation. In the universe today, 13.8 billion years after the Big Bang, many are in dense clusters of tens, hundreds, or even thousands of galaxies. 

However, these clusters have not always existed, and a key question in modern cosmology is how such massive structures assembled in the early universe. 

Pinpointing when and how they formed should provide insight into the process of galaxy cluster evolution, including the role played by dark matter in shaping these cosmic metropolises. 

Now, using the combined strengths of Herschel and Planck, astronomers have found objects in the distant universe seen at a time when it was only 3 billion years old that could be precursors of the clusters seen around us today.

Planck’s main goal was to provide the most precise map of the relic radiation of the Big Bang, the cosmic microwave background. To do so, it surveyed the entire sky in nine different wavelengths from the far-infrared to radio in order to eliminate foreground emission from our galaxy and others in the universe. 

But those foreground sources can be important in other fields of astronomy, and it was in Planck’s short-wavelength data that scientists were able to identify 234 bright sources with characteristics that suggested they were located in the distant early universe. 

Herschel then observed these objects across the far-infrared to submillimeter wavelength range but with much higher sensitivity and angular resolution. 

Herschel revealed that the vast majority of the Planck-detected sources are consistent with dense concentrations of galaxies in the early universe, vigorously forming new stars. 

Each of these young galaxies is seen to be converting gas and dust into stars at a rate of a few hundred to 1,500 times the mass of our Sun per year. By comparison, our Milky Way Galaxy today is producing stars at an average rate of just one solar mass per year. 

While the astronomers have not yet conclusively established the ages and luminosities of many of these newly discovered distant galaxy concentrations, they are the best candidates yet found for “protoclusters” — precursors of the large mature galaxy clusters we see in the universe today. 

“Hints of these kinds of objects had been found earlier in data from Herschel and other telescopes, but the all-sky capability of Planck revealed many more candidates for us to study,” said Hervé Dole of the Institut d’Astrophysique Spatiale, Orsay. 

“We still have a lot to learn about this new population, requiring further follow-up studies with other observatories. But we believe that they are a missing piece of cosmological structure formation.” 

“We are now preparing an extended catalog of possible protoclusters detected by Planck, which should help us identify even more of these objects,” said Ludovic Montier from the Institut de Recherche en Astrophysique et Planétologie, Toulouse. 

“This exciting result was possible thanks to the synergy between Herschel and Planck: rare objects could be identified from the Planck data covering the entire sky, and then Herschel was able to scrutinize them in finer detail,” said Göran Pilbratt from ESA. 

“Both space observatories completed their science observations in 2013, but their rich datasets will be exploited for plentiful new insights about the cosmos for years to come.” 

First pair of merging stars destined to become a supernova found

This artist’s impression shows the central part of the planetary nebula Henize 2-428. The core of this unique object consists of two white dwarf stars, each with a mass a little less than that of the Sun. They are expected to slowly draw closer to each other and merge in around 700 million years. This event will create a dazzling supernova of Type Ia and destroy both stars. Image credit: ESO/L. Calçada

Astronomers using ESO facilities in combination with telescopes in the Canary Islands have identified two surprisingly massive stars at the heart of the planetary nebula Henize 2-428. As they orbit each other the two stars are expected to slowly get closer and closer, and when they merge, about 700 million years from now, they will contain enough material to ignite a vast supernova explosion. The results appeared online in the journal Nature on 9th February 2015.

The team of astronomers, led by Miguel Santander-García (Observatorio Astronómico Nacional, Alcalá de Henares, Spain; Instituto de Ciencia de Materiales de Madrid (CSIC), Madrid, Spain), has discovered a close pair of white dwarf stars — tiny, extremely dense stellar remnants — that have a total mass of about 1.8 times that of the Sun. This is the most massive such pair yet found and when these two stars merge in the future they will create a runaway thermonuclear explosion leading to a Type Ia supernova.

This image of the unusual planetary nebula Henize 2-428 was obtained using ESO’s Very Large Telescope at the Paranal Observatory in Chile. Image credit: ESO

 

 

The team who found this massive pair actually set out to try to solve a different problem. They wanted to find out how some stars produce such strangely shaped and asymmetric nebulae late in their lives. One of the objects they studied was the unusual planetary nebula known as Henize 2-428.

“When we looked at this object’s central star with ESO’s Very Large Telescope, we found not just one but a pair of stars at the heart of this strangely lopsided glowing cloud,” says coauthor Henri Boffin from ESO.
This supports the theory that double central stars may explain the odd shapes of some of these nebulae, but an even more interesting result was to come.
“Further observations made with telescopes in the Canary Islands allowed us to determine the orbit of the two stars and deduce both the masses of the two stars and their separation. This was when the biggest surprise was revealed,” reports Romano Corradi, another of the study’s authors and researcher at the Instituto de Astrofísica de Canarias (Tenerife, IAC).
They found that each of the stars has a mass slightly less than that of the Sun and that they orbit each other every four hours. They are sufficiently close to one another that, according to the Einstein’s theory of general relativity, they will grow closer and closer, spiralling in due to the emission of gravitational waves, before eventually merging into a single star within the next 700 million years.
The resulting star will be so massive that nothing can then prevent it from collapsing in on itself and subsequently exploding as a supernova. “Until now, the formation of supernovae Type Ia by the merging of two white dwarfs was purely theoretical,” explains David Jones, coauthor of the article and ESO Fellow at the time the data were obtained. “The pair of stars in Henize 2-428 is the real thing!”
“It’s an extremely enigmatic system,” concludes Santander-García. “It will have important repercussions for the study of supernovae Type Ia, which are widely used to measure astronomical distances and were key to the discovery that the expansion of the Universe is accelerating due to dark energy”.